Now let us see how the “coherer,” as the filings tube is called, is used in actual wireless telegraphy. [Fig. 33a] shows a simple arrangement for the purpose. A is an induction coil, and B the battery supplying the current. The coil is fitted with a spark gap, consisting of two highly polished brass balls CC, one of these balls being connected to a vertical wire supported by a pole, and the other to earth. D is a Morse key for starting and stopping the current. When the key is pressed down, current flows from the battery to the coil, and in passing through the coil it is raised to a very high voltage, as described in [Chapter VIII]. This high tension current is sent into the aerial wire, which quickly becomes charged up to its utmost limits. But more current continues to arrive, and so the electricity in the aerial, unable to bear any longer the enormous pressure, takes the only path of escape and bursts violently across the air gap separating the brass balls. Surging oscillations are then produced in the aerial, the ether is violently disturbed, and electric waves are set in motion. This is the transmitting part of the apparatus.
a. Transmitting. b. Receiving.
Fig. 33.—Diagram of simple Wireless Transmitting and Receiving Apparatus.
If a stone is dropped into a pond, little waves are set in motion, and these spread outwards in ever-widening rings. Electric waves also are propagated outwards in widening rings, but instead of travelling in one plane only, like the water waves, they proceed in every plane; and when they arrive at the receiving aerial they set up in it oscillations of the same nature as those which produced the waves. Let us suppose electric waves to reach the aerial wire of [Fig. 33b]. The resistance of the coherer H is at once lowered so that current from battery N flows and operates the relay F, which closes the circuit of battery M. This battery has a twofold task. It operates the sounder E, and it energizes the electro-magnet of the de-coherer K, as shown by the dotted lines. This de-coherer is simply an electric bell without the gong, arranged so that the hammer strikes the coherer tube; and its purpose is to tap the tube automatically and much more rapidly than is possible by hand. The sounder therefore gives a click, and the de-coherer taps the tube, restoring the resistance of the filings. The circuit of battery N is then broken, and the relay therefore interrupts the circuit of battery M. If waves continue to arrive, the circuits are again closed, another click is given, and again the hammer taps the tube. As long as waves are falling upon the aerial, the alternate makings and breakings of the circuits follow one another very rapidly and the sounder goes on working. When the waves cease, the hammer of the de-coherer has the last word, and the circuits of both batteries remain broken. To confine the electric waves to their proper sphere two coils of wire, LL, called choking coils, are inserted as shown.
In this simple apparatus we have all the really essential features of a wireless installation for short distances. For long distance work various modifications are necessary, but the principle remains exactly the same. In land wireless stations the single vertical aerial wire becomes an elaborate arrangement of wires carried on huge masts and towers. The distance over which signals can be transmitted and received depends to a considerable extent upon the height of the aerial, and consequently land stations have the supporting masts or towers from one to several hundred feet in height, according to the range over which it is desired to work. As a rule the same aerial is used both for transmitting and receiving, but some stations have a separate aerial for each purpose. A good idea of the appearance of commercial aerials for long distance working may be obtained from the frontispiece, which shows the Marconi station at Glace Bay, Nova Scotia, from which wireless communication is held with the Marconi station at Clifden, in Galway, Ireland.
In the first wireless stations what is called a “plain aerial” transmitter was used, and this was almost the same as the transmitting apparatus in [Fig. 33a], except, of course, that it was on a larger scale. This arrangement had many serious drawbacks, including that of a very limited range, and it has been abandoned in favour of the “coupled” transmitter, a sketch of which is shown in [Fig. 34]. In this transmitter there are two separate circuits, having the same rate of oscillation. A is an induction coil, supplied with current from the battery B, and C is a condenser. A condenser is simply an apparatus for storing up charges of electricity. It may take a variety of forms, but in every case it must consist of two conducting layers separated by a non-conducting layer, the latter being called the “dielectric.” The Leyden jar is a condenser, with conducting layers of tinfoil and a dielectric of glass, but the condensers used for wireless purposes generally consist of a number of parallel sheets of metal separated by glass or mica, or in some cases by air only. The induction coil charges up the condenser with high tension electricity, until the pressure becomes so great that the electricity is discharged in the form of a spark between the brass balls of the spark gap D. The accumulated electric energy in the condenser then surges violently backwards and forwards, and by induction corresponding surgings are produced in the aerial circuit, these latter surgings setting up electric waves in the ether.
Fig. 34.—Wireless “Coupled” Transmitter.
For the sake of simplicity we have represented the apparatus as using an induction coil, but in all stations of any size the coil is replaced by a step-up transformer, and the current is supplied either from an electric light power station at some town near by, or from a power house specially built for the purpose. Alternating current is generally used, and if the current supplied is continuous, it is converted into alternating current. This may be done by making the continuous current drive an electric motor, which in turn drives a dynamo generating alternating current. In any case, the original current is too low in voltage to be used directly, but in passing through the transformer it is raised to the required high pressure. The transmitting key, which is inserted between the dynamo and the transformer, is specially constructed to prevent the operator from receiving accidental shocks, and the spark gap is enclosed in a sort of sound-proof box, to deaden the miniature thunders of the discharge.